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Battery charger question

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Myke

Wed Jul 04 2007, 11:06PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

This schematic is from the book "Electricity And Electronics". I was wondering if this works and if it does then could I sub the TIP 31 for a 2N3055? The equation for the resistor that isn't R1 is 600mV/desired charging current in mA = resistor in ohms.

Myke

Wed Jul 04 2007, 11:17PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Sorry about the double post.

Sulaiman

Thu Jul 05 2007, 01:18PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

That circuit looks very poor.
Q1 needs to pass the main charging current (e.g. 1.5A)
R1 has to pass at least (charging current)/(Q1 current-gain) e.g. 1.5A/30 = 50mA
So VR1 = 1k x 50m = 50V !!!!! (and 50V x 50mA = 2.5Watts)

Use a high-gain darlington for Q1 or a mosfet or an igbt.
OR
search for a circuit diagram that someone has actually used successfully.

What do you want to charge, and from what power source?

Myke

Thu Jul 05 2007, 04:59PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I wanted to charge a 12V 7AH lead acid battery. I would use a 14.4V power supply using a rewound MOT. I tried to find something better but I only found car battery chargers.

Electroholic

Thu Jul 05 2007, 07:05PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

if you the SLA is for cycle use, you can charge it all the way up to 14.5V or even 15V no problem.
if its just for standby use, then 13.5 is safe.

also any big enough 12V wallwart will work 500mA to 1A ish, just look at the output current and work out the charge time.

Sulaiman

Thu Jul 05 2007, 11:51PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I have a 12V 7AH SLA (now renamed VRLA) battery, for general purpose use.
it is just the latest of many, I like to always have one available.

Good maintenance is worth it,
Go to the Yuasa website (or the one for your battery) and look at the specifications for a charger!

The 'fully charged' voltage is temperature dependent, try not to over- or under-charge the battery.
A simple transformer - rectifier - (optional capacitor) - resistor will do just fine.
Measure voltage with your meter and stop charging when 'full'

You could 'trickle charge' at 10mA continuously.

Steve Conner

Fri Jul 06 2007, 08:32AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

There is a simple circuit you can make using a LM317 regulator as a current source, IIRC.

I use a 6V one of these gel batteries to drive a set of super powered halogen lights for my bike in winter. I always just charged it overnight, off a current limited lab power supply set to 7.2V and 0.5A. On first plugging in, it draws the full 0.5A and pulls the power supply down to 6V, but by the morning the voltage goes up to 7.2V and the current dies away to practically nothing.

It never gets used in summer, so I charge it up every month or two to make sure it doesn't go stale. I got 5 years use out of the last battery before it got too weak to last the ride to work and back, so I guess I was treating it right. I thought that was pretty good for a $20 battery! It was an actual Yuasa brand battery, though. I don't know if the no-brand Chinese ones for ride-on toys are so good.

You could always make a current limited power supply, since it'll be useful for a load of other things. It's probably not a good idea to use a car battery charger, since these little gel batteries aren't as tough as car batteries. It would probably overvolt the battery and boil it dry.

Avalanche

Sat Jul 07 2007, 07:26PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

The first circuit on this page

is a much better charger. Probably the one Steve was on about, it has current limiting so it will act as a current source when you're trying to exceed the rated current, but it will act as a voltage regulator when you're under the current set point. That makes it perfect for charging SLAs. I believe you want some kind of voltage regulator really, to prevent the terminal voltage from rising to 'gassing' voltage if you leave it on too long. I'm building that circuit with a switch to switch between 14.4v (for fast/bulk charging) and 13.65v (for continuous trickle charging).

An even simpler charger I'm using at the moment is simply a 7815 regulator with 2 ordinary diodes on the output, to drop the output to about 13.8 volts. That works quite well also. A fully charged SLA will have a terminal voltage of 12.9 volts after it has been taken off charge and left standing for a while.

Myke

Sat Jul 07 2007, 08:16PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Thank you for the help.

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